1. Field of the Invention
The present invention relates generally to back biasing of systems on a chip, and more particularly to fine grain voltage scaling of back biasing with voltage control devices distributed across a common substrate of distributed devices.
2. Description of the Related Art
Low power consumption is emerging as a significant feature for electronic devices, particularly handheld and/or battery powered applications. Significant reductions in leakage and power consumption can be obtained by reverse back biasing transistor devices of the circuitry of the electronic device. Reverse back biasing (RBB) generally involves driving a voltage differential between the bulk (or body) connection and source terminal of the device to increase its threshold voltage (VT). In a standard complementary metal-oxide semiconductor (CMOS) configuration, the source and substrate of P-channel MOS (PMOS) devices are both tied to VDD and the source and substrate of N-channel MOS (NMOS) devices are both tied to VSS. In one conventional reverse back biased CMOS configuration, the body connections of PMOS devices are instead pulled to a voltage above VDD and the body connections of NMOS devices are instead pulled to a voltage below VSS. A negative charge pump or the like may be used to drive the substrate of NMOS devices to a negative voltage level below VSS. VSS is assumed to be zero (0) Volts (V) or ground, where it is understood that VSS may be any other positive or negative voltage level establishing a reference supply voltage level.
RBB has been a very effective technique for certain semiconductor process technologies, including 130 nanometer (nm) and 90 nm designs. RBB has become more problematic and less popular with the more advanced process technologies including 65 nm and below, such as including 65 nm, 55 nm, 40 nm, 28 nm, etc. In general, for most process technologies including the more advanced process technologies, RBB may result in a relatively high leakage reduction of low threshold voltage (LVT) devices with relative low performance impact, and may result in a significant leakage reduction of standard threshold voltage (SVT) devices without substantial performance impact. LVT and SVT devices are generally faster with higher performance but are also more leaky than high threshold voltage (HVT) devices. HVT devices are typically used in areas in which lower performance is needed. RBB has less impact on performance with greater impact on leakage reduction for LVT and even SVT devices, yet has caused a significant performance decrease of the lower leakage HVT devices, particularly for the more advanced process technologies.
Since the application of a uniform level of RBB on HVT, SVT and LVT devices can cause an unacceptable performance impact on the HVT devices, one solution has been to replace HVT devices with either SVT or LVT devices. Such substitution, however, defeats the very purpose of leakage and power reduction. Another solution has been to isolate the LVT and SVT devices from the HVT devices into separate and isolated substrate domains. Such isolation techniques, however, may cause a significant level of inconvenient location re-design, and require a significant increase in overhead by adding guard rings or the like to establish and ensure domain isolation. The use of RBB, therefore, has become more limited with the more advanced process technologies at 65 nm and below because of its performance impact.
Back biasing may be performed in the forward direction, which is referred to as forward back biasing (FBB), to improve performance at the cost of increased leakage and power consumption. FBB involves driving a voltage differential between the bulk (or body) connection and source terminal of the device in the opposite direction of RBB relative to the supply voltages to decrease its threshold voltage (VT). FBB also has an opposite effect as that of RBB, in which the transistor device becomes faster at the cost of increased leakage and power consumption. The application of a uniform level of FBB on HVT, SVT and LVT devices also causes a significant performance impact on the HVT devices as compared to the SVT and LVT devices.